There are two sets of aims in this parts of the program project. The first set concerns the CD4/gp120 interaction and attendant conformation changes in gp120. The second set concerns crystallographic analyses of gp120/CD4 complexes and forms of truncated gp160 trimers, prepared as described in part 1. (1) Phage display has been used to identify a group of peptides that inhibit CD4 binding with gp120. The interactions of these peptides with gp120 will be characterized using fluorescence polarization, surface plasmon resonance, circular dischrosm, effects on mAb binding, and related approaches, in order to identify the key determinants of binding/inhibition, to discover tighter-binding peptides, and to determine whether peptide binding affects V3 loop conformation. Crystals of gp120/peptide complexes will be sought. A screen for peptide inhibitors will be undertaken, using bead-based chemical libraries biased by information from the peptide studies. (2) X-ray structural analysis of HIV envelope components will be undertaken, using crystals obtained through the efforts in part 1. Complexes that will be investigated included gp120/CD4 Fab (using gp120 with homogenous, short oligosaccharides expressed in Lec 3.2.8.1 cells) and trimeric ectodomain of gp160. Complexes of small-molecule ligands with gp41, identified by NMR methods in project 3, will also be examined crystallographically.